设计模式之装饰者模式

好文：https://www.cnblogs.com/chenxing818/p/4705919.html

1. 一个示例

类的继承关系：

1.1 定义

public interface Person {
    void eat();
}

public class Man implements Person {
    public void eat() {
        System.out.println("男人在吃");
    }
}

public abstract class Decorator implements Person {
    protected Person person;
    public void setPerson(Person person) {
        this.person = person;
    }
    public void eat() {
        person.eat();
    }
}

public class ManDecorator1 extends Decorator {
    public void eat() {
        super.eat();
        reEat();
        System.out.println("ManDecorator1类");
    }
    public void reEat() {
        System.out.println("再吃一顿饭");
    }
}

1.2 测试

public class Test {
    public static void main(String[] args) {
        Man man = new Man();
        ManDecorator1 md1 = new ManDecorator1();
        md1.setPerson(man);
        md1.eat();		//先执行super.eat();即man.eat();，后执行reEat();
    }
}

打印：
男人在吃
再吃一顿饭
ManDecorator1类

1.3 总结

装饰者模式的作用：动态地扩展对象的功能（理解：动态主要体现在执行super.eat()时，super可动态地绑定其他对象，此处为man）

2. Android系统中的装饰者模式

类的继承关系：
public class Activity extends ContextThemeWrapper {}
public class ContextThemeWrapper extends ContextWrapper {}
public class ContextWrapper extends Context {}
class ContextImpl extends Context {}

发现也符合装饰者模式，由示例可知有setPerson函数将Man的实例化对象保存到ManDecorator1的成员变量中，因此也有一个函数将ContextImpl的实例化对象保存到Activity的成员变量中
在分析Activity的启动流程中：

frameworks\base\core\java\android\app\ActivityThread.java
public final class ActivityThread {
		private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
				ContextImpl appContext = createBaseContextForActivity(r);
				activity.attach(appContext, this, getInstrumentation(), r.token,						----------------			//appContext为ContextImpl实例化对象
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,                   |
                        r.embeddedID, r.lastNonConfigurationInstances, config,                     |
                        r.referrer, r.voiceInteractor, window, r.configCallback);                  |
		}                                                                                              |
}                                                                                                  |
                                                                                                   |
frameworks\base\core\java\android\app\Activity.java                                                |
public class Activity extends ContextThemeWrapper                                                  |
        implements LayoutInflater.Factory2,                                                        |
        Window.Callback, KeyEvent.Callback,                                                        |
        OnCreateContextMenuListener, ComponentCallbacks2,                                          |
        Window.OnWindowDismissedCallback, WindowControllerCallback,                                |
        AutofillManager.AutofillClient {                                                           |
		final void attach(Context context, ActivityThread aThread,		<---------------------------------
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, String referrer, IVoiceInteractor voiceInteractor,
            Window window, ActivityConfigCallback activityConfigCallback) {
				attachBaseContext(context);		------------------------------------------------------------
				...                                                                                      |
		}                                                                                            |
		protected void attachBaseContext(Context newBase) {		<---------------------------------------
        super.attachBaseContext(newBase);		-----------------------------------
    }                                                                         |
}                                                                             |
                                                                              |
frameworks\base\core\java\android\view\ContextThemeWrapper.java               |
public class ContextThemeWrapper extends ContextWrapper {                     |
		protected void attachBaseContext(Context newBase) {		<--------------------
        super.attachBaseContext(newBase);					-------------------------------------------------------
    }                                                                                                   |
}                                                                                                       |
                                                                                                        |
frameworks\base\core\java\android\content\ContextWrapper.java                                           |
public class ContextWrapper extends Context {                                                           |
		protected void attachBaseContext(Context base) {		<------------------------------------------------
        if (mBase != null) {
            throw new IllegalStateException("Base context already set");
        }
        mBase = base;					//ContextWrapper.mBase为ContextImpl实例化对象，Activity继承自ContextWrapper，所以Activity.mBase为ContextImpl实例化对象
    }
}

因此：

Activity,Service,Application都是ContextWrapper的子类，ContextWrapper里面有一个Context类型的成员变量mBase，当然它实际的类型是ContextImpl
执行Activity.mBase.***()相当于ContextWrapper.mBase.***()，最终执行ContextImpl.***()